Flight Level (FL) is used to denote altitude above which threshold, and how is it expressed?

• A. Altitude expressed in feet above sea level.
• B. A measure of altitude (in hundreds of feet) used by aircraft flying above 18,000 feet with the altimeter set at 29.92 inches of mercury.
• C. The indicated climb rate at sea level.
• D. A measure of airspeed.

Answer: (B)

Flight Levels are a way to express altitude using a standard pressure setting. Above the transition altitude, pilots set the altimeter to 29.92 inches of mercury, and the altitude is shown in hundreds of feet. That means FL180 equals 18,000 feet. The purpose of this is to keep vertical separation consistent regardless of local pressure variations, which is why the value is given as a pressure-based height rather than a local ambient altitude. So the threshold is the transition altitude (18,000 feet in the U.S.), and the elevation is expressed as a number of hundreds of feet with the 29.92 inHg setting.